It is possible to worship a form of the Lord made of physical elements such as earth, water, pulp, wood and metal. In the forest one can make a form with no more than earth and water and worship Him according to the above principles. A devotee who has full control over his self should be very sober and peaceful and must be satisfied simply with eating whatever fruits and vegetables are available in the forest.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
लब्ध्वा द्रव्यमयीमर्चां क्षित्यम्ब्वादिषु वार्चयेत् । आभृतात्मा मुनिः शान्तो यतवाङ्मितवन्यभुक्
labdhvā dravyamayīmarcāṃ kṣityambvādiṣu vārcayet | ābhṛtātmā muniḥ śānto yatavāṅmitavanyabhuk
SB 4.8.56