How do i unify imported parts with surfaces?

I am new to 3d modeling and, now, I am wondering to make a control for our robots. I participate in Warthog Robotics here in Brasil, one group of USP (University of de São Paulo) and we need to make this control based on Dualshock 4 V2. But we need to change the dimensions to place our electronics. I found one 3d model here at grab cad, and this model is impossible to edit on it, so I am doing some cuts and making surfaces, cut and surface... Sorry for my terrible English, but can you help me?

Attached I sent a part of the model...

2 Answers

Hi , your file is a single solid body. What do you really need ?

Hi ! first you can offset top surface to 0.5 (see offset image capture), I did'nt offset all the surfaces, then use fill hole in surface menu, then use fill surface (tangent) for the rest of holes, knit and thick to 1mm.