So I printed a case for the LCD on the A8. It was actually my second print of it since my belts were loose and the first version was out of whack. The second version was perfect, but the back acrylic piece was too big! Apparently Anet makes design changes willy nilly and ships whatever.
Undaunted, I imported the case file into Tinkercad and used it to design a brand new back piece that snaps into place (so its multilayered). I also incorporated a space for a 40mm fan if I decide to add it. Took me about 20 minutes total to knock out the design.
When I want to test fit before doing the entire print, I'll just print out the first couple layers or however much is needed to check clearances.
If the part that needs test fit isn't on the bottom, just block off the rest of the design with a giant "box of empty space" in tinkercad.
The test parts are usually only 10 minute print jobs.
I bought some gauge blocks and got all psyched up to calibrate my printer, but it ended up being good out of the box.
I still have to play the high/low game with super precise parts, but I guess that's just the nature of the beast.
Buy a set of digital calipers if you don't already have them. They make designing useful parts in tinkercad much quicker.