Custom or customized 'mechs?

[Inxentas]

This is a bit of an open question. As a player I'm coming from heavily codified game systems where there is only one way to go about some stuff. In BT that is different and I'm a bit lost when it comes to the idea of custom mechs. I understand that people tend to use custom mechs outside of the tournament scope, and I've found refit rules in Campaign Operations as well as Tech Manual's construction rules. My question is... and players are just as welcome to answer this... how do you handle custom 'mechs in pickup games? Or rather, pickup games in which you and your opponent have agreed custom mechs are a thing.

Do you use CO rules and go with "whatever still adheres to the construction rules"? Or is recreating the mech using Tech Manual the way to go? Which of these two sources are most used?

I'm a little lost because I can imagine a player thinking along the lines of I like X mech, if only it had Y weapon. I can also imagine a player wants to create one from scratch. If you aren't managing refits through CO, how would you usually go about custom(ized) mechs, rule-wise?

I want to open up creative possibilities to my fellow players but I don't want them to bring completely ridiculous broken builds.

[cray]

Speaking for my personal games over the last 38 years: my friends and I always used pick-up games to test out custom designs. We usually hash out what's acceptable for the session - Clan tech only, UrbanMechs only, 10,000 BV with any technology, 3025 Inner Sphere only, super heavy designs only, etc., and build per appropriate rule books.

In campaigns we generally stick to whatever designs are available and, later, salvaged. As the force gets sufficient technical knowledge and cash, they'll customize within the rules and whatever restrictions the GM thinks are appropriate.

[Inxentas]

Thank you (and everyone that DM-ed me with answers to my questions)!