So I started buying parts knowing the last time I looked at a commercial vent booster it cost $100.. I've now realized it's dropped to $60 (and it has air detection and fan control):
https://www.amazon.com/AC-Infinity-Register-Thermostat-Control/dp/B0792QR5YT/ref=sr_1_2?keywords=AC+Infinity+AIRTAP+T4%2C+Quiet+Register+Booster&qid=1562095135&s=gateway&sr=8-2

However, some are still complaining of noise, plus the DIY version is still half the price (and customizable).

So I've included both a STEP file and individual STL files. You're going to need Sixteen 8-32 x 1-1/2 screws, equal amount of nuts, three fans, and a speed controller:

https://www.ebay.com/itm/AC-to-DC-3-12V-2A-Adjustable-Power-Supply-Adapter-Motor-Speed-Controller-US/153522995098?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

https://www.ebay.com/itm/Adaptec-ACC-9700-3-x3-80mm-3-Pin-Computer-Case-Fan-5000-RPM-2127300/300469881915?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

https://www.mcmaster.com/90480a009

https://www.mcmaster.com/90272A203

I sourced three fans off ebay, each that run off 12v @ .25A (controller handles up to 2A). Each fan can run up to 5,000 rpm with a 53CFM (so 160CFM total output, which is technically better than the 100CFM commercial version on amazon).

I haven't built this yet but I'm confident in the design (will update after assembly). Should be able to wire up all three fans up the to controller (Red and black, yellow isn't needed).

Each grill half should be about a 3 hour print job, roughly 70 cents in material each. Support shouldn't be needed. There are nut pockets included so you should only need a phillips screwdriver to build. I split it in half because I assume most don't have a printer big enough to print an 11" long part.