I doubt there is anything but is there any jobs/volunteer for Mtb trail building I'm already a member of singtraction but I wanna do it more!!!
Where are you based?
If you're in Singletraction then you're already part of one of the most active and successful groups in the country. You could approach someone like Clixbys but I think H&S plus child protection would put a dampener on any firms taking you on.
Take the opportunity to learn all you can, try and get your head round the aspects of trail design (positive and negative control points, qualifier sections, rolling crown switchbacks etc.)
BTCV active in your area? could gain lots of experience and transferable skills by helping with them.
Thanks that's my dream to work for clixbys/upm till hill