I designed this in Fusion 360. It is 6mm thick, 95mm long, 74mm wide, and weighs 20 grams. The slots are sized for Futaba S3003-type servos, with little notches at either end for the wires to clear as they slide in. Other standard servos will fit. (Tried Hitec and Airtronics.) There is a very fine (paper thin) gap around the perimeter of the mounted S3003 servo. The holes are perfect for 2.5mm machine screws to self-thread. As added insurance, you can press nuts (2.5 mm nuts; the hex fits a 5mm socket/wrench) into the hex-shaped indentations on the bottom. 2.5x12mm screws will tighten so the bottom end is flush for any clearance issues underneath. I printed my tray with Hatchbox PLA and epoxied it to 1/8" x 1/4" basswood rails.
The aircraft I designed this for is a traditional balsa/rag .40 glow powered "Das Ugly Stik" type.
I sliced the tray in Cura and printed on a Monoprice Select Mini V1 at 50mm/sec with 33% infill, .175mm layer height, 1.2mm wall thickness, 3 wall line count, .72mm top/bottom thickness with 5 top and bottom layers. I print on a heated glass bed. Orient the model on the print bed with the hex-shaped holes facing upwards. No raft or skirt should be necessary for this model.