Lexus: 2000 RX 300 - Rear Seat Belts - How to make my child safe ?

Hello everyone:

Wonder if others have experienced the same issue. This problem is not
specific to the RX or even Lexus as such.

During long rides, my 6 year old, like any other little kids, gets
restless and cannot stay in a sitting position. She would
occassionally like to lie down and sleep. We have her keep her belts
on while she is lying down, however, the belts do not look safe enough
for me for that position.

Any ideas on how to make her more secure ? She is not using the
booster seat anymore.

Maybe I am paranoid, but I guess it is better to be paranoid than be
sorry later.

Thanks for your ideas.
-- Rick