Print this upside down. When you print the retainer, make sure the tapers on the holes are up. Use 100% infill for the retainer but the main part could be 20% or more.
1 Frame - $2.71 with 30% infill and $20 per KG.
1 RetainerA - $0.60 of PLA at 100% infill.
1 RetainerB - $0.60 of PLA at 100% infill.
8 M3x16mm flat-head screws. 14 or 18mm will also work.
Philips (way cheaper):
24 M3x10mm flat-head screws.
91294A130 or 92125A130
Also consider parts from www.boltdepot.com
Need 32 of M3 brass inserts that are 4.1mm OD and 3mm long.
I am not sure how liquid-tight PLA is, but you could always apply a layer of resin to it, and then cure that into the PLA to form a seal. Or dip or spray with Polyurethane.
This will use $3.50 of filament if you pay $20 per KG.
I am using PLA because it is the stiffest and has the least warp. You can spray it with clear paint to seal it.