Oxalic acid vapourising in Polystyrene hives

Has anyone done this? I have Paynes Polystyrene hives and am very pleased with them. I want to use my Varox vapouriser to treat the hives and make sure I haven't got too much varroa. I used to rely on counting mite drop but have been told that this can be unreliable and that others who thought they had little or no varroa have discovered they had a lot when they used the vapouriser. From what I've read online I think others have put the tray under the OMF. This would mean inserting the varox tray from the back of the hive and blocking up the back as well as the front entrance.
Any advice would be gratefully received

Welcome to the forum Mary.
I think you're mixing up two activities ... counting mites and treating them. The former is inaccurately done by monitoring mite drop (as you say). Better to use an alcohol or icing sugar 'roll' on 300 bees in a pot and count the mites displaced.
Treating mites in poly hives can be done from underneath - those who do will likely be along shortly to describe exactly how. I treat via the front entrance, but have a different type of vaporiser ...

PS Also worth searching the beekeeping forum as there are a lot who use Varrox on there and they have created some ingenious trays to hold the machine in place.

Thanks for the advice Fatshark. I had worked out that you have to put the tray under the OMF for a Polystyrene hive but thank you for confirming that. No I'm not confusing counting and treating. I was acting on advice from an experienced beekeeper. I've kept bees continuously for 40 years so perhaps I'm experienced but he's more active and is commercial whereas I only have 2 or 3 hives and really do it because I enjoy it and am interested in the conservation of pollinators so grow things for them in my garden. If I get some honey its a bonus. We didn't have Varroa here for many years after everyone else had it so I'm not very experienced with diagnosis and treatment. I did the Varrox treatment yesterday morning. I used the garden lawnmower tractor as a battery and cut the grass around the hives at the same time. I put the Varrox tray on a sheet of cardboard covered in tinfoil under the OMF and sealed the back as well as the front entrance with cloths. It was very easy but you do need an escape plan to get away from the vapour! I checked the mite drop this morning (24 hours later) - only 1 in the stronger hive but 100 in the weaker hive. So its just as well I did the Varrox treatment as I was seeing no mite drop in the weaker hive. Its a lesson learned and I now believe that the Varrox method is useful for diagnosis as well as treatment. Having read more on various forums I'm planning to do more Varrox treatments - 2 more at 5 days apart which should apparently reduce the mites to a very low level. I will then use Varrox at the more normal time in winter when there is no brood.