I got a mast boot from WaterBoot.com. I have a sunbrella cover over that. They may have dimensions for the 382, I don't remember. And I do not know if, but suspect that, the mast dimensions are the same on the 383.
The mast collar on my 382 is not deep enough to make the installation of the boot easy, but it is doable with care and patience. I use SparTite for the partners. That too takes care so it does not dribble down the mast and sets up evenly. If possible, you want the SparTite to come out with the mast when you unstep the mast and the shape of the collar also makes this difficult. Again, the collar is not as deep as it could/should be. I don't remember how I dealt with that. Maybe I put lube on the mast itself, so the SparTite will stay in the boat.
Another thing: The 382 step (and perhaps the 383) is not well designed. It is a standing lip that the mast slips over. But both parts are aluminum. They are constantly wet from the water coming down the mast in the rain. Even though anodized, over time they corrode and weld themselves together. The mast will not come up. This last time I coated everything with TefGel to try to prevent that. I wish I had used Duralac on one of the surfaces before I put on the TefGel. I have not pulled the mast in 4 years, so I don't know if the TefGel has prevented the corrosion/welding.
I recommend you join Attainable Adventure Cruising and read John Harries discussion on SparTite, WaterBoot, and rig tuning. I have been sailing for decades and I still learn things from John.