I have put together a few pics which show the mast support - not very well I must admit. I can't get a good photo as the cover is over the boat however I think is shows the general idea.
The key dimensions are : support tube diameter = 27mm ( depends upon transom fittings) Overall length of support from foot/end of tube to Centre line of roller 1170 mm. The locking plate is sized 30 x 70 mm set 365mm above foot/end of support tube but to suit you transom fittings.
The roller is a standard trailer keel roller with bracket such as :
Photos
Park 21 mast support.pdf
(268kb) downloaded 62 time(s).
Hope this helps
I am also new to the parker 21 having purchased mine for the 2020 season but only getting a few weeks use which I very much enjoyed, hope you too are pleased with your boat.
, regards Rob Pierce