I use plywood templates in a variety of ways. But first I use a 3 hp plunge router with speed control and an extended base made of ribbed plexiglass. My router bits are top bearing wide diameter 1.5 in. with a downshear(downshear prevents surface tear out). The bits are relatively short so I have installed an after market quick release chuck that has effectively increased the bit length. The bit must be wider than the chuck to be effective. Of course the wider bit requires a slower operating speed.
Making templates is easy with top bearing bits, since there is no offset from the cutting edge to the bearing, the cut is identical to the template dimensions. Layout the template to the housing size, cut out inside the housing lines with a jig saw, nail sqaure 3/4 stock to the housing layout and make the final cut with the router. Remove the 3/4, the template is finished.
With S4S planed timber, the template can be clamped or screwed directly to the timber, the router depth can be set to uniform depth. Non-uniform timber require special care and individual settings. Square tubular template frames can be effective on non-uniform timber.
I have also used multiple housings on a single long template that align to a reference mark, saving layout detailing.
Consider making a master template, that is used to produce working templates, since there is some risk of template damage during work.
Anyhow, what do you need to do?