Resetting tub after new tile

Hello all,
What a great site for the homeowner/DIYer!
Heres my issue...
I removed a whirlpool tub (deck mounted) to replace tile. The new tile resulted in a height difference from floor to finished deck. Previously, there was an irregular shaped piece of plywood under the tub but did not provide full support. The tub has a 24" x 46" piece of OSB attached to the bottom that support the pump and is sloped to the front using firring strips in the back and middle.
What would be the best way to 'set' the tub?
I was considering screwing two (2) 1/2" piece of plywood together and then attaching to the bottom (under OSB) and then placing in a mortar bed.
Does this sound doable?
Can the mortar be placed in plastic bags to keep it away from the subfloor and tub bottom?
Do I need to dry fit and mark area for mortar?
If I use the two (2) 1/2" pieces of ply, I would need ~ 1/2" of mortar at the highest point to obtain a fairly level installation.
Any thoughts/suggestions?

You can use plastic to isolate the mortar, plaster of paris, or stucolite (sp?) under it. You don't need to add any more ply - in fact, my opinion is you'd be better off not and let the mortar fill in between the current shims. Spread out enough so you get good coverage, set the tub, mush it down until it is both level and the desired height, then leave it overnight, or at least a few hours before you do anything else to it.

Mortar in plastic bags.
Does it set up that way?

Maybe, if it's a chemical reaction.

I put small piles of mortar under the tub, and let the tub squish it down.
One time, I tried to plane it out evenly, Yeah, one time.
I had to pull the tub back up, and use less mortar.

Resetting tub in mortar bed

Hey Terry,
You bring up a good question. The OSB on the bottom has two firring strips.. one at the rear (~3/4") and one in the middle (~1/2") and none in the front. Would you put the mortar just in these areas to set the tub? I decided to screw the two 1/2" pieces of plywood to the floor instead od the tub because I really didn't want to try to make a mortar bed ~1-3/4" thick.
This way I am only trying to raise the tub ~1/2" with the mortar and ensuring I have full support underneath and not on the rim.

Tub set in mortar

Hello all,
I screwed the plywood to the floor in the approximate area the tub would sit. I oversized these pieces to give myself some wiggle room when setting the tub. I followed Terry's method and placed piles of mortar where the shims on the OSB would fall and then some smaller piles in between. Installed the tub and 'squished' it down until I just made contact with the rim. I had a few areas where there was a 1/16 to 1/8 gap where my tile was not exactly flat but that was easily taken up with a small bead of caulk all the way around. Installed new fixtures and checked plumbing for leaks and so far , so good.

Also, I did place a layer of plastic down over pieces of plywood before laying the mortar. I then layed another piece after the mortar to keep the tub off of direct contact. Everything set up as expected and I hope this will allow easy removal if ever needed in the future. (not planning on it!!!!)

Thanks everyone for the help.