private_repo.list

# /etc/apt/sources.list.d/private-repo.list
deb [arch=amd64 signed-by=/etc/apt/trusted.gpg.d/private_repo.gpg] file:/usr/local/mydebs ./

private_repo.sh

#!/bin/bash

# choose a key to sign the repo. I'm using my email
my_key="[my-email-address]"

# get key into apt-key
gpg --armor --export "${my_key}" > /tmp/key.asc
sudo apt-key add /tmp/key.asc && rm /tmp/key.asc

# set up repo folder
if [[ ! -d /usr/local/mydebs ]]; then
    mkdir /usr/local/mydebs
fi
cd /usr/local/mydebs || exit 1

# move downloaded rstudio deb into repo folder, e.g.
mv ~/Downloads/rstudio-1.0.44-amd64.deb .

# generate Packages file
dpkg-scanpackages . /dev/null > Packages
gzip --keep --force -9 Packages

# generate Release file
apt-ftparchive release . > Release

# Clearsign Release file
gpg --clearsign --digest-algo SHA512 --local-user "${my_key}" \
    -o InRelease Release

# add repo to sources.list "deb file:/usr/local/mydebs ./" >> /etc/apt/sources.list

# add to apt
sudo apt update

# install Rstudio!
sudo apt install rstudio

exit 0

update_repo.sh

#!/bin/bash

set -eux

# choose a key to sign the repo. I'm using my email
my_key=twharrop@gmail.com

# remove the files if they're there
rm -f Packages.gz Packages Release InRelease

# generate Packages file
dpkg-scanpackages . /dev/null > Packages
gzip --keep --force -9 Packages

# generate Release file
apt-ftparchive release . > Release

# Clearsign Release file
gpg --clearsign --digest-algo SHA512 --local-user "${my_key}" \
    -o InRelease Release

# add repo to sources.list "deb file:/usr/local/mydebs ./" >> /etc/apt/sources.list

# add to apt
sudo apt update

The key can be added to apt like this:

gpg --armor --export [key_id] \
| sudo apt-key add -

Move a key to the new format (see https://askubuntu.com/questions/1407632/key-is-stored-in-legacy-trusted-gpg-keyring-etc-apt-trusted-gpg)

sudo apt-key export [key_id] | sudo gpg --dearmour -o /etc/apt/trusted.gpg.d/[key_id].gpg