No resources, but I found the easiest thing was to get agreement that FOSS components could be used to develop the software, subject to suitable licensing, and

No resources, but I found the easiest thing was to get agreement that FOSS components could be used to develop the software, subject to suitable licensing, and that any required changes to FOSS components could be contributed upstream. By tying those two things together, I've had minimal pushback. Anything I wanted to use across projects I then released as FOSS, and as it was enhanced over use in multiple projects, those enhancements were contributed back per my agreement. This requires a little investment up front to create the initial codebase, but I found that it's generally worthwhile to do something the first time for a client, and then rewrite it as the FOSS version, because then the open source version could avoid all the mistakes I'd made in the first prototype :-)