Well, there are two reasons why I would not use Apex: Weight and Bulk. The biggest reason to use it would be the MYOG aspect. It is generally easy to sew when making a quilt…no baffles, no filling each baffle, no closing each baffle.
Closing off the bottom will help slightly with bugs. But it also turns your quilt into a technical sleeping bag (your goal, I believe.) Big Agnes did this and incorporated a second shell layer to hold the pad in place underneath you. It worked fine, but, it also ended up fairly heavy.
If you do this, you also need a zipper. This will usually prohibit using the warmer 20F bag in high summer when it is actually too much. Opening the bottom and top will provide an avenue for bugs. So you also need an enclosed tent. Again, the weight starts building up, and the overall versatility drops. You would find the 20F quilt way too warm in an enclosed tent for summer use, so, a 40F bag (another MYOG project) would also be needed.
I would suggest skipping the under sheet, it will get in the way. Only in the shoulder seasons will it be useful. So, as a design, I would look at EE quilts and incorporate some light 1/2″ or 3/8″ straps, rather than the Big Agnes like full panel. And make one with an enclosed foot box, easily kicked off your feet in summer through the open back allowing some lowering of the temp rating. And allowing the straps to be unhooked and simply laid over you, again, easy to kick off/fold down as needed.
Whatever you do, I would opt for a slightly wider and slightly longer version. Quilts tend to be “airy”. The extra size will let you pull it around you closing it up thoroughly in colder weather. And, the extra bulk of any extra clothing you might need (extra socks, insulated pants and a jacket) is still effective if the weather turns nasty needing the extra insulation.
Apex is roughly equivalent to 500-600FP down. Typically, the difference between a 20F 850fp down quilt and a 20F synthetic quilt is over a half pound (between 10-14oz or 280-400g.) The biggest reason for choosing a quilt over a bag is WEIGHT. They do away with unneeded under insulation. Your rough design does this but does not save weight over a standard 850fp down quilt. It will save dollars over down, though.
Nor does it save BULK. It will pack up to two to three times the size of a standard down quilt so you will need a somewhat larger and heavier pack to carry it. Anyway, it will suffer in durability over a 5 year span. It will go from a 20F quilt to a 30F quilt over that time period, maybe loosing more than that if you stuff it tightly into your pack. Synthetics do not take stuffing very well.
And, the MYOG project won’t save you any dollars over just purchasing an entry level (500-600fp down) bag. Small scale MYOG projects are usually nearly as expensive as buying some item, complete. You might pay yourself about $1/hr for the labour involved in making one.
An overriding use for a synthetic bag would be in wet conditions, though. They do not suffer from being damp or wet nearly as much as down. But you mention the Sierra’s. Eventually, you will be replacing all the gear you inherited. There is always the satisfaction of something you made yourself, too. Both dollar expenditure, MYOG experience and self satisfaction will influence your later gear choices.
Do not sell yourself short. I would go for a lighter 40F Apex quilt to start with, with plans to extend the temp with my other gear as needed. Make one to refine your technique. And use the heavy bag you have if the weather will turn nasty. Use it to refine the features you want. Then, after a year of use, decide what else you need for comfort in the shoulder seasons. For example: a 1/2″ thick sweater, a down jacket, insulated pants, and two pair of wool socks will likely take a 30F quilt below 20F and you will know if you are carrying all that weight more than once or twice per year…ergo, you might have need of a warmer bag.
