on most pressure bleeders there's a clamp style thingie, it did it's job ok for me several times in the past put some rags around the BMC to stop potential brakefluid running down your enginebay and get it over with as quick as possible
I used it on an my old Toyota worked a treat no air bubbles and very good pedal feel. I have changed my fluid to Castrol SRF bleed the brakes using a one man bleeding kit with a one way valve and the brakes are spongy with still some air in the system. Previously I had a helper so wasn't an issue.
I've tried what RJT suggested, using my trusty gunson ezibleed, I have a spare cap with a union fitted but unfortunately the cap doesn't actually seal to the reservior fully by design (the top of the reservoir has notches). So you don't get good pressure and sometimes you get fluid passing the cap seal. I also tried the optional clampy kit that you can get for the ezibleed kit but that failed for the same reason.
I tend use a hand vacuum pump at the caliper bleed nipple which works great.
That's the same kit I have a gunsons one. Shame about the cap not being airtight, how about wrapping a loat of ptfe tape around the thread and then putting the cap on? will need to take a better look at the cap and fitting.
I didn't use a cap, I bled mine with a clamp-on universal fitting for my pressure bleeder (don't have pictures on my phone but they're in one of my old threads about pressure bleeding). Worked a treat!