That’s genius. Just a couple of plastic waste pipe fittings. Superb. 
Even when split it is not certain that water/dirt will ingress as if there is still grease present it will protect the joint. The only way to tell is to remove the old boots, clean out the old grease and inspect the ball joints for wear and condition. As you need to disconnect the ball joints if it is to replace the boots or the complete joint this is no hardship. I have replaced lower swivel boots on mine as well as the track rod end ones. Recently when I had Autolink waxoyl mine they found top swivel boots split so they supplied and fitted for me at the same time. Mazda supply the boots as a genuine part so are obviously happy for them to be replaced as a perishable part.
All
I am going with these as replacement dust boots, I assume they fit the MX5 as the statement “MX5 bottom ball joint. Mk1 Mazda MX5 lower and upper ball joints.”
has in the auction. They are a lot cheaper for 4 than one of our club sponsors are selling them for each.
http://www.ebay.co.uk/itm/360500251483?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
I am awaiting delivery and will provide feedback on size and fit
The approximate hole sizes are 30 mm and 16 mm, Uncompressed height 26 mm.
This does equate to the sizing on S Wales Propshaft as a DC155K
http://www.southwalesprops.co.uk/propshaft/consumables/ball_joint.htm
I have replaced the lower boots on mine and Andrew at Autolink changed the top ones when it was in for some other work. When they split/perish they do not generally leave a large gap for water/grit to get in and the grease tends to protect anyway. So providing they have not been split for too long it is likely the joints wil be fine and can be cleaned and regreased before fitting new boots. x 4 boots will cost about £40.00 and then either fit yourself or have them fitted by a garage. Autolink may be too far away for you being about I guess an hour and a half away but any reputable garage should be able to carry out the job.