Adding Mouthfeel to a thin Russian Imperial Stout???

Back in early April I made an attempt to brew my first RIS. I had some questions/concerns which one of our members addressed well. After letting it sit on 2oz of medium toast oak for the last few months I'd like to bottle now. My concern at this point is that it fermented lower than I wanted it to and it seems a little astringent and thin. So my plan is to add some maltodextrin and lactose at bottling to help give if more mouthfeel and smooth it out.
I have a little over 4 gallons of RIS in the carboy now. My question would be how much maltodextrin and lactose to add. 2oz of each? 4oz of one and not the other? Am I way off here?
I'm less concerned about the astringency since I'm fairly confident the tannins will drop out over time, I just think its too thin. BTW I was shooting for ~37 FG and I got around 28 FG (98 SG).
Thanks in advance for any feedback.

I had a similar situation happen to a RIS I had brewed. 1 unrehydrated pack of US-05 brought a 1.098 beer to 1.018 and it definitely was too thin for my tastes. I dumped 8oz of lactose and 8oz of maltodextrin directly to the carb'd keg (probably 4.5gallons) and somehow managed to get the lid back on before the geyser of foam came rolling out

For my tastes, this was just about right and no one who's tasted it has thought it was chewy or thick at all. Your milage may vary, but I certainly wouldn't think that 4oz of each would be too much.

I wouldn't say that FG 1.028 , down from 1.098, is exactly low, but I've faced this too and solved it by back adding lactose. I'd suggest making little mini-samplers and carbonating with a carbonator cap. Then start with the equivalent of 8oz lactose, or alternatively 4 oz of each. Adjust to taste.

I found that doing this with lactose adds an artificial-sweetener taste that will diminish after a couple months.

Thanks for your feedback hoppus and SpeedYellow!
I was wanting to add 4oz of both but thought it might be too much. They both came in 4oz packets and I hate storing partially used ingredients; especially when I know I won't be using it again any time soon.
Couple other questions:
1. Can/should I throw a packet of us-05 in my priming bucket or should I use something like a champagne yeast?
2. I also wanted to add a little Maker's Mark in there too. What would be a reasonable amount? I want to sense the bourbon but I don't want it to be overpowering. I was thinking of starting with maybe 8oz, tasting, then adding a little more if I think it needed it.
Any suggestions? Thanks!

8oz is probably right on the money. Generally speaking, I've heard 6-12oz is a standard amount to add. I soaked 4oz of oak in 20oz of knob creek for 3 months and added all of it the RIS I did, but I imagine that would probably be to much for most people's tastes. I just happen to like knob creek a lot I think .

I'll have to defer on the carbonation question, I never had any luck with carb'ing high % brews. Part of the reason I switched to kegging actually.