I am a big fan of Union Station. Many of the cars they make are available no where else, and while the process does require patience, is fairly simple.
I have several that are half done and if I can find the right box, will post an "in progress" shot or two later on. The one thing I noticed between the ones I bought ?12? years ago, and ones manufactured more recently is that the laser cutting is much improved, and the recent ones required very little clean up around the windows.
The windows are provided as a cut out from an acrylic sheet. The rest of the acrylic sheet IS the reinforcement for the side (assuming you are using an ALM core kit). The key-for me at least- to the process is to follow their instructions to the letter, and use the spray glue they recommend. Basically, you lay out the acrylic base sheet- you need the correct side up in order to have the windows line up right, spray the sheet and back of car side, let the glue dry BEFORE putting it together, attach the car side to that (get everything aligned before putting any pressure on it- the spray glue works like a contact cement, and you have), press it together. Cut and fit the fluting (if it is a fluted car, then glue it on, but you will only put the glue on the back of the fluting, and not on the side, so you want to get it on before it has a chance to dry completely. They also provide some small strips of fluting that are used smooth side up to model letterboards and numberboards on fluted cars.
I use low tack masking tape mounted adhesive side up on a board to hold the styrene pieces while applying spray glue. That one I learned the hard way, and I think there may still be a couple of parts of my first car in the basement of my old house, glued in a corner somewhere.
If you are building a fluted car, be sure you have an exacting method to cut small pieces of styrene at very precise 90 degree angles and precise lengths- any error becomes very apparent at the doors.
Do remember the one other rule...if you actually complete a car- interior, grabs, roof and underbody detail, paint, lettering, etc., a major manufacturer will announce production within 24 hours of the last bit drying.
