Once all the dirty work is done it is time to put the pieces into the mod. If your finished acrylic is a window this can be done in several ways with Silicone, H-Lock trim, Screws or bolts.
Silicone in a easy use tube
H-Lock trim uninstalled
Fan screws are great for mounting windows
Silicone is the easiest of the three to use as it just takes applying, placing and pressure till it sets. Setup time is usually printed on the tube or package and varies with the type, temperature and humidity it is used in. Place a small bead, about a 1/8", around where it needs to be affixed. For bigger pieces a small circle may be made with the sealant. This acts as a suction cup when pressure is applied.
Silicone installed & the window pressed down
Once the piece is set into place use a soft cloth with weights placed on top until the sealant sets. If the piece is not straight it can be removed once the silicone drys. Remove the window and peel the silicone off. Then do it all over again.
Installed dry window with silicone
Screws, bolts and nuts can be used to hold down pieces to the metal. These are probably the best methods to use if weight or vibration is a factor such as fan spacers, brackets and grills. There is more layout involved when using fasteners, as the alignment is a major factor with installing your window. When using screws the piece your going through needs a hole larger then the threads and the piece it is screwing into needs a hole slightly smaller then the threads.
Processor heatsink installed with bracket, all thread & cap nuts to the backing plate
This may be one of the harder ways to do windows, but it can have an amazing result. The H-Lock trim has been done, H-Lock Trim Window, so I won't repeat it.
Lastly I will mention glues or solvents for plastics. They are meant for joining same type plastics together. This requires skill, patience and knowledge. Solvents aren't really a starting place and are mentioned only as part of learning to master plastics in modding.